I. Field of the Invention
The invention relates to the field of glass ceramic composite structures and methods for forming composite glass ceramic structures using a low temperature joining process.
II. Description of the Prior Art
Glass ceramics, which generally consist of an inorganic, non-porous material having a crystalline phase and a glassy phase, are known for specialized applications. Such glass ceramics are manufactured by selecting suitable raw materials, melting, refining, homogenizing, and then hot forming the material into a glassy blank. After the glassy blank is cooled and annealed, a temperature treatment follows whereby the glassy blank is transformed into a glass ceramic by controlled volume crystallization (ceramization). Ceramization is a two-step process; nuclei are formed within the glass at one temperature, and then grown at a higher temperature. The dual structure of the glass ceramic material can impart very special properties, including a very low coefficient of thermal expansion (CTE).
One preferred material, Zerodur(copyright) (available from Schott Glass Technologies, Duryea, Pa.) contains about 65-80 weight percent crystalline phase with a high quartz structure, which imparts a negative linear thermal expansion. The remaining glassy phase (which surrounds the crystals) has a positive thermal expansion. The resulting behavior from the negative-CTE crystalline phase and the positive-CTE glassy phase is a material with an extremely low CTE.
Glass ceramics are useful in a wide variety of applications, such as mirror substrates for astronomical telescopes; mirror substrates for X-ray telescopes in satellites, optical elements for comet probes, weather satellites, and microlithography; frames and mirrors for ring-laser gyroscopes; distance gauges in laser resonators; measurement rods as standards for precision measurement technology, and other uses where very low CTE is important.
Large segments of monolithic glass ceramic are often used for many of the applications listed above. However, these large segments of monolithic glass ceramic are often very massive. For instance, larger astronomical telescopes can contain mirrors that exceed 3.6 meters, and the appropriate glass ceramic for use in such telescopes can exceed several tons. Thus, there is a need to develop light-weighted glass ceramic materials to overcome the problems associated with the massive nature of large monolithic segments of glass ceramic.
Various joining methods for optical materials are known; e.g. heat fusion, or frit bonding, however, none provides a low-temperature solution such as is provided for in the present invention.
The known prior art for fabricating light-weighted blanks (i.e. heat fusion, frit bonding) suffers from several drawbacks. For example, pressure and temperature are typically required to form strong joints at high temperature. Developing loading/unloading fixtures for operation at T greater than 600xc2x0 C. is complex and expensive. Further, heat fusion and frit bonding processes are conducted at temperatures near or above the glass transition temperature (Tg) of the starting material (i.e., glassy Zerodur(copyright), or ULE). The viscosity of glass is sufficiently low at these temperatures, such that limited flow (deformation) can occur. This deformation can cause gross dimensional changes, which can yield a defective mirror blank.
Additionally, large, high temperature furnaces (up to 2.0 m in diameter) with stringent thermal tolerances are required for heat fusion and frit bonding. Such furnaces must be custom-made and are often very expensive. Furthermore, glassy Zerodur(copyright) shrinks by approximately 3% during ceramization. This shrinkage can cause joint stresses that result in deformation and/or catastrophic failure of the mirror blank during joining.
Yet another difficulty encountered in the prior art is that light-weighted mirror blanks can fail during high temperature joining when thermally induced stresses form at the joint interfaces (especially during cooling to room temperature). Such joint failure results in a 100% loss after a tremendous amount of value has been added to the mirror blank (i.e., machining, polishing, water-jet cutting, assembly, high temperature fixturing, etc.).
A solution to this problem is needed, to allow for the low-temperature joining of fabricated mirror blanks in a step-wise manner.